Wanted: Cruise speed mark on the ASI tape

[Eric_Greenwell]

My Phoenix U15 touring motorglider has a 110 knot TAS cruise. The IAS value for that 110 knot value varies greatly from sea-level to 18000 feet, so I'd like a "tick mark" or other indication on the ASI tape, so I know the desired IAS for that 110 knot cruise. Currently, I have to find/look at the tiny TAS reading at the bottom of the tape.

Hey, you put in a TAS Vne indication (for which I am very grateful), where the red line on the tape adjusts for TAS, so this ought to be easy!

 

[dynonsupport]

Eric,
An aircraft that has a cruise speed specified in TAS isn't something we've run into before, and when I glanced at the POH for the U15 I didn't see it either. What's the purpose of maintaining a specific TAS in cruise?

 

[jnmeade]

Could you put a Va adjusted for weight on the speed tape?
How about IAS markings for those that use IAS for Vne?

 

[Eric_Greenwell]

Eric,
An aircraft that has a cruise speed specified in TAS isn't something we've run into before, and when I glanced at the POH for the U15 I didn't see it either. What's the purpose of maintaining a specific TAS in cruise?

I don't think the POH specifies a cruise speed, but owners typically use 110 knots TAS. In stable air (not rising or descending), that's 5000 rpm on the Rotax 912. Because it's only 10 knot less than the 120 knot TAS Vne for the Phoenix, it takes more care to avoid exceeding Vne.

On autopilot, rising air will cause the plane to speed up as the AP tries to maintain altitude. Glancing at the speed tape doesn't give me the information easily, as I have to look for the little TAS indicator at the bottom. At a constant altitude, I can determine the equivalent IAS  to watch for, but it's not as good as having a mark that is correct at any altitude. Especially when descending, I find myself flying too slowly.

It's not necessarily a safety problem on autopilot, as SkyView will "bump" up the nose if the airspeed gets close to Vne (thank you for that!), but I find it's a nuisance when hand flying and changing altitudes, particularly in the 10,000' to 16,000' range, as I often do in the high desert areas of NV, UT, etc.

 

[RVDan]

Does Skyview have a method to do Vmo?

 

[dynonsupport]

Eric,
Please forgive my questions- to decide the priority of a feature request, one of the things we need to determine is how many customers it is useful for, and a TAS cruise marker is not an operating limit I am used to so it's hard for me to judge.

As an LSA, you couldn't put this new firmware and feature in the plane without Phoenix's approval. I looked at the Phoenix POH again and it specifically says all speeds are in Knots IAS. Vne is 120 knots IAS. So I'm still confused why operators limit themselves to 110 TAS when that seems much more conservative than the manufacturer's requirements. If there is a flutter issue, the Vne in IAS is supposed to be set such that at the service ceiling, the Vne IAS does not lead to a TAS that is an issue. Given that, it seems unlikely that Phoenix would approve such a marking on the EFIS and approve the installation.

Am I missing something?

 

[dynonsupport]

Does Skyview have a method to do Vmo? 

SkyView allows your Vne to be indicated in IAS or TAS, so a lot like a Vmo. However, the question here is for another pointer below Vne that is a TAS pointer for an arbitrary speed.

 

[Eric_Greenwell]

As an LSA, you couldn't put this new firmware and feature in the plane without Phoenix's approval. I looked at the Phoenix POH again and it specifically says all speeds are in Knots IAS. Vne is 120 knots IAS.

The original manuals did not make it clear the Vne is 120 knots TAS, but later ones do. This TAS Vne is not unusual for gliders, which is how the Phoenix is certified in the SLSA category. In fact, the TAS Vne that SkyView now provides was at the request of a Phoenix owner [smiley=happy.gif] , and we are very happy to have it! To remain under 120 knots TAS at 18,000' (there is no altitude limit for the Phoenix when flown by a pilot with a glider rating), for example, would require an IAS Vne of 88 knots. That 88 knots IAS is fine at 18,000 feet, since you are trucking along at 120 TAS, but using that 88 kt/IAS Vne at 4500' (for example), you'd be "trucking along" at only 95 knts/TAS!

There is no firmware that needs Phoenix's approval, as they do not supply the EFIS. I just want a TAS marker on the airspeed tape that I can set for whatever value I wish.